Electromagnetic interactions

The electromagnetic interaction is described by a gauge theory headed by the symmetry group whose density of the Lagrangian can be written as:

where is the electromagnetic field tensor and is the gauge covariant derivative, with indicating the covariant four-potential of the electromagnetic field, generated by the charge ; is the external field due to an eventual external source; is the coupling constant that coincides with the electric charge of the bispinor field.

Things get more interesting when one considers the electroweak unification..